Planetary cores are considered to be composed of iron (Fe) and light elements. Knowledge of structure and properties of liquid Fe-light element alloys under high pressures is essential to discuss the nature and dynamics of planetary liquid cores such as Earth's outer core, while those remain poorly understood. In this article, we introduce our recent studies about the influence of possible light elements [silicon (Si), carbon (C), and sulfur (S)] on the structure of liquid Fe, and the relation between the structure and physical property of liquid Fe-light element alloys.